The invention relates to a system for lightning and surge protection of objects, in which equipment is set up which has an electrical power supply. More specifically, the present invention relates to a system for surge protection of an object comprising a supply unit, which is connected to the secondary output of an external transformer, in which the secondary windings of the external transformer are coupled in a star configuration, the star point of the transformer being connected to a downstream neutral conductor and an earth electrode provided near to the transformer.
In particular, the invention may be used as a system for lightning and surge protection for an object set up on a limited area, on the ground or on a building. Examples of such an object comprise an installation provided with an antenna, e.g. a GSM base station. Surge protection devices for electrical power supplies are generally known.
The protection device according to the application may be used to protect against surges with a high energy content, such as surges caused by lightning or electromagnetic pulse (EMP). More in particular, the application relates to protection against surge caused by lightning strikes in a power supply for electrical equipment set up in objects, such as transmitter/receiver stations for radio traffic.
For such a protection device, in addition to a number of specific components and measures, one or more (preferably at least two) earth electrodes are employed for the purpose of deflecting the charge which is inherent in the surge and distributing it over the greatest possible area. It goes without saying that these earth electrodes must have the least possible resistance to the zero potential. It is, moreover, important that the ground area over which the charge of the lightning strike is to be distributed is at least of a minimum magnitude.
E.g., in the case of electrical power supplies for base stations for mobile telecommunications, such a minimum area is often not available because, for economic reasons, the area is preferably chosen to be no larger than necessary for the dimensions of the foot of the antenna mast. In the case that the object is positioned on top of a building, usually only a limited number of conductors with earth electrodes are used.
If the charge of a lightning or of EMP strikes the cabinet in which the power supply is housed or the metal frame to which the cabinet is attached, there is a danger of parasitic flash-over of the charge to the electrical conductors of the power supply. Since this charge is dissipated relatively poorly, the voltage in the power supply can rise to such an extent that flash-over can damage the components of the power supply, such as switches or cause failures of the power supply. Also, other equipment of the object, such as the equipment being supplied with power may be damaged.
That this voltage can be substantial can be understood from the fact that from a direct lightning strike a peak current of as much as 150 kA may arise, which must be deflected via an earth electrode having an impedance of 2.5 Ohm (this value being a standard value for earth electrodes, in practice this value may be higher or lower).
Momentarily, this may lead to voltages of over 100 kV. For such a peak voltage, a power supply for low voltage applications is not equipped.
Such a parasitic flash-over is prevented according to the state of the art by connecting surge protective devices, such as varistors or spark gaps, between the frame and each of the phases and between the frame and each of the neutral conductor of the power supply. The frame is connected directly to an earth connection, such as one or more earth electrodes. This way, parasitic flash-over from the part on which the strike occurs to one of the conductors is prevented.
In this known solution, however, it can not be prevented that in the power supply substantial damage occurs when a direct lightning strike occurs on the frame, which will be further explained in the detailed description. Here, it suffices to mention that this damage may comprise the burning of one or more components of the power supply caused by the very large currents. Moreover, mechanical damage may arise in the power supply as the large currents flowing through the conductors of each of the phase conductors and of the neutral conductor cause the connecting conductors to be pulled from the connection points, through the electromagnetic fields caused by the large currents, as a result of which an interruption in the current flow occurs.
It need not be mentioned that, also because of the earlier mentioned periphery arrangement of the power supply and the less proper accessibility thereof, repair of the damage will take a lot of time. As a result, the installation powered by the power supply will be out of service for a longer period of time, which leads to a higher risk of operational damage.
European patent application EP-A-0 128 344 describes an arrangement for surge arresters in a high voltage transformer. In this arrangement, surge arresters are connected between each phase conductor and the neutral conductor, and also between the neutral conductor and an earth electrode. The surge arresters are all of the same type. Additionally, a capacitor is connected between the neutral conductor and a second earth electrode. This results in a protection of the transformer against too high voltage peaks, in which the capacitor controls the dynamic behavior of the surge arresters. The surge arresters are usually chosen to be spark gap elements, as these can be used in high voltage applications. The arrangement described is meant specifically for protection of the high voltage transformer.
A disadvantage of the use of spark gap elements or spark gaps connected between the phase conductors and the neutral conductor is that a rest voltage results which is poorly defined and usually too large. Furthermore, spark gaps will keep an undefined rest voltage, dependent on the rise time of the lightning pulse, which may be 2.5 to 4 kV, which is too high for low voltage equipment. Also, the spark gap elements cause a short circuit and thus a net following current, which almost certainly results in breakdown of the fuses (of the electricity provider). Breakdown of the fuses results in operational down time of the equipment supplied by the transformer.
The present invention seeks to provide a system for surge protection for use in low voltage applications, which does not have the disadvantages of the known systems described above. The present invention also seeks to provide a solution to the problem that the power supply defined in the preamble above, has such a limited deflection path to earth in order to deflect the charge of the strike, that the peak voltage occurring is relatively high and decreases relatively slowly.